JP2004176170A - Method for producing molten iron - Google Patents
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【０００１】
【発明の属する技術分野】
本発明は溶鉄の製法に関し、より詳細には、鉄鉱石などの酸化鉄源を炭材等の炭素質還元剤と共に加熱還元し、鉄分純度の高い溶鉄を効率よく製造し得るように改善された方法に関するものである。
【０００２】
【従来の技術】
鉄鉱石等の酸化鉄源を還元し溶鉄を製造する方法として現在実用化されているのは、高炉−転炉法が主流であるが、この方法は、還元剤としてコークスの使用が不可欠であり、しかもスケールメリットを追求するあまり、経済動向に対する生産柔軟性を欠き、特に多品種・少量生産への対応に問題がある。
【０００３】
一方、小規模で多品種・少量生産向きの製鉄法として、MIDREX法に代表される直接製鉄法がある。しかしこの方法は、還元剤として天然ガスを必要とするので、当該設備の立地条件に制約がある。
【０００４】
これらに対し、石炭ベースの炭素質還元剤を用いて還元鉄を製造し、該還元鉄を電気炉で加熱溶融することにより溶鉄を製造する方法としてＳＬ／ＲＮ法があり、最近では回転炉床炉と電気溶解炉を結合し、酸化鉄の還元と生成する還元鉄の加熱溶解を一貫して行う直接製鉄法も多数提案されている。しかし、これらの方法には大量の電力を必要とするので、当該設備の立地条件が電力供給事情の良い場所に限定される。
【０００５】
上記の様な状況の下で、鉄鉱石等の鉄源と石炭等の炭素質還元剤を用いて溶鉄を製造する溶融・還元製鉄法の改良研究が盛んに進められており、その代表例として、予備還元炉と溶融還元炉を組み合わせたDIOS法やHIsme1t法が提案されている。これらの方法を実用化する上で重要となるのは、溶融還元炉で高レベルの２次燃焼率と着熱効率を確保することであるが、これらを高めると、鉄鉱石等の鉄源中の脈石成分に由来して加熱還元時に副生するスラグ内に高濃度の酸化鉄（ＦｅＯ）が混入し、処理炉の内張り耐火物を著しく溶損するという問題がクローズアップされてくる。こうした問題の対応策として、炉体を水冷し耐火物の溶損を抑制する方法も提案されているが、炉体からの熱損失が大きくなるため、溶鉄の生産性や熱エネルギー効率に多大な悪影響を及ぼす。
【０００６】
また直接製鉄法の一つとして、鉄鉱石等の鉄源と炭材などの炭素質還元剤を混合して成形した炭材内装成形体（ペレットやブリケットなど）を回転炉床炉で加熱・還元し、溶融還元炉で最終的に溶融還元を行って溶鉄を製造する方法が知られている（特許文献１，２，３など）。これらの方法には、溶融還元炉で生成する高温の排ガス熱を回転炉床炉へ導入して有効利用することで、設備全体としての熱効率を高める狙いがあり、それなりの効果が期待される。ところが、溶融還元炉から抜き出される高温の排出ガスには多量のダストが含まれており、これが配管内に付着・堆積するばかりでなく回転炉床炉の炉壁などにも付着・堆積し、安定操業の障害となる。
【０００７】
しかも、溶融還元炉で熱変動が起ると、回転炉床炉へ供給される高温ガスの熱量や還元ポテンシャルが変動し、設備全体としての操業状況を不安定にする。そして操業状況が不安定になると、回転炉床炉で進行する酸化鉄の還元効率や金属化率が変動し、製品鉄の純度が不安定になるばかりでなく、副生スラグ内への酸化鉄（ＦｅＯ）の混入量が増大し、炉床耐火物の溶損を招くことになる。
【０００８】
更に溶融還元法では、溶融還元炉に多量の酸素と熱を加えるため、炉体耐火物の補修や吹込み羽口のメンテナンスが不可欠で、そのためには、炉体を傾動したり移動させるための設備が必要となり、これら付帯設備の設置や耐火物補修のための経済的負担は、溶鉄の製造コストに少なからぬ影響を及ぼす。
【０００９】
【特許文献１】
特公平３−６０８８３号（特許請求の範囲、図１など）
【特許文献２】
特開２００１−２７９３１３号（特許請求の範囲、図３など）
【特許文献３】
特開２００１−２４７９２０号（特許請求の範囲、図１など）。
【００１０】
【発明が解決しようとする課題】
本発明は上記の様な状況に着目してなされたものであって、その目的は、酸化鉄と炭素質還元剤を含む混合物を原料とし、回転炉床炉と溶解炉を組み合わせた溶鉄製造プロセスにおいて、それらの操業条件を適切に制御することにより、回転炉床炉や溶解炉の耐火物の溶損を可及的に抑制しつつ、鉄分純度の高い溶鉄を生産性よく製造することのできる方法を提供することにある。
【００１１】
【課題を解決するための手段】
上記課題を達成することのできた本発明に係る溶鉄の製法とは、酸化鉄源と炭素質還元剤を含む原料混合物を加熱還元炉へ装入し、該混合物中の酸化鉄を炭素質還元剤により還元して固形還元鉄とした後、該固形還元鉄を溶解炉へ送り、燃料として供給される炭材を燃焼させることにより、該溶解炉で前記固形還元鉄を溶解させて鉄溶湯を得る溶鉄の製法において、
前記固形還元鉄の金属化率を６０％以上に高めてから溶解炉へ送り、該溶解炉へ供給する酸素と炭材の量を制御することによって、該溶解炉内におけるＣＯガスの２次燃焼率を４０％以下に制御するところに要旨が存在する。
【００１２】
なお本発明において溶解炉内の２次燃焼率とは、溶解炉からの排出ガスを連続的にサンプリングし、得られるガス成分の分析値から下記式によって算出される値であり、
２次燃焼率＝100×（CO2＋H2O）／（CO＋CO2＋H2＋H2O）
また着熱効率は、溶解炉からの排出ガス温度および鉄溶湯温度の測定値と、上記式によって求められる２次燃焼率を用いて算出される。
【００１３】
本発明で使用する前記原料混合物を調製するに当っては、炭素質還元剤および前記炭材の量を、該炭素質還元剤および前記炭材中の揮発分を除いた炭素量（Ａ）が、［（当該混合物中の酸化鉄の還元に必要な化学当量）＋（溶鉄製品中の目標炭素濃度分）＋（固形還元鉄の溶解に必要な熱量分）］以上となるようにコントロールすれば、原料混合物中の酸化鉄分の固体還元から還元溶融、更には溶融金属鉄の取得に亘る一連の工程をより効率よく円滑に遂行できるので好ましい。
【００１４】
上記炭素量（Ａ）の調整は、加熱還元炉へ装入される前記混合物中へ配合する炭素質還元剤、加熱還元炉で製造された還元鉄中へ配合する炭素質還元剤、および前記溶解炉へ供給される炭材から選ばれる１つ以上によって行うことができる。
【００１５】
また、前記溶解炉へ供給する酸素含有ガスとして、酸素濃度が９０％以上の高純度酸素を使用すれば、溶解炉内における２次燃焼効率が高められるばかりでなく、２次燃焼時の燃焼温度や溶融鉄浴への着熱効率の制御も一層容易になり、更には発生ガス量が抑えられてダスト発生量も低減できるので好ましい。該高酸素濃度ガスの溶解炉への供給は、底吹き、上吹き、横吹きの何れか、又はこれらの任意の組み合わせによって行うことができるが、これらの中でも、高酸素濃度ガスをスラグ層に向けて上吹きし或いは横吹きすると、添加される炭材をスラグ層内で効率よく燃焼させることができ、着熱効率を高めることができるので好ましい。
【００１６】
尚、溶解炉は固定式または転動式とし、該溶解炉への前記固形還元鉄や炭材およびスラグ成分調整用のフラックスを、前記溶解炉の上方から炉内へ重力落下により投入し、もしくは溶湯内へ吹き込む方式を採用すれば、還元溶融を簡単な操作で効率よく進めることができるので好ましい。この際、溶解炉の鉄溶湯内に不活性ガスを吹き込んで撹拌すれば、固形還元鉄の溶解が一段と加速され、処理時間を短縮できるので好ましい。
【００１７】
固定式の溶解炉を使用する場合、当該溶解炉の側壁に設ける溶鉄と溶融スラグの取出し用タップホールを、その開口高さ位置が不活性ガスの吹抜けを起こさない位置に設定しておけば、ガス吹き込みによる吹込み羽口部の閉塞事故を未然に回避できるので好ましい。
【００１８】
なお、本発明で使用する前記酸化鉄源としては鉄鉱石が最も一般的であるが、この他、ミルスケールなどを使用することも勿論可能であり、更には高炉ダストや転炉ダストの如き酸化鉄含有ダスト、更には酸化鉄と共に非鉄金属やその酸化物を含有するもの、例えば、ニッケル、クロム、マンガン、チタンなどの非鉄金属やその酸化物を含有する鉱石や、金属精錬設備から排出されるダストやスラグ等を使用することも可能である。
【００１９】
尚、上記非鉄金属やその酸化物の場合は、溶鉄を製造する過程で発生するスラグにこれらを移行させ、純度の高い非鉄金属やその酸化物の製造原料もしくは製品として回収することができる。
【００２０】
前記溶解炉で固形還元鉄を溶解する際には、炭材などに由来して溶融金属鉄中に相当量の硫黄が混入してくるが、この溶解工程で適量のＣａＯ含有物質を添加し、該溶解炉内で生成するスラグの塩基度（CaO／SiO2）が１．２以上となる様に調整してやれば、溶融金属鉄中の硫黄分を溶融スラグ方向に移行させることができ、金属鉄の硫黄含量を低減できるので好ましい。この際、溶融金属鉄の炭素含量が２％以上となる様に、溶解炉へ添加する炭材の量を調整してやれば、硫黄分のスラグ方向への分配比が高まり、溶融鉄中の硫黄含量を一段と効率よく低減できるので好ましい。
【００２１】
更に、２次燃焼熱の鉄溶湯への着熱効率は６０％以上に高めることが好ましい。
【００２２】
加熱還元炉で得られる固形還元鉄は、そのまま高温を維持したまま直接溶解炉へ投入することで、固形還元鉄の保有熱をその加熱溶解に有効に利用できるので好ましいが、設備上の制約によっては、該固形還元鉄を一旦ヤード等に保管しておき、必要に応じて溶解炉へ送って加熱溶解を行うことも勿論可能である。
【００２３】
なお上記方法を実施する際に、前記溶解炉で発生する燃焼ガスは相当の熱を保有しているので、加熱還元炉へ送って熱源として有効利用することもできるが、この際には、送給配管や加熱還元炉へのダスト付着の問題を回避するため、該燃焼ガスを冷却・除塵し、該ガス中のダスト含有量を５ｇ／Ｎｍ3以下に抑えるのがよく、また、前記加熱還元炉からの排ガスを用いて空気を予熱し、当該加熱還元炉の燃焼用空気、原料混合物の乾燥、更には当該原料混合物の原料となる酸化鉄源や炭素質還元剤の乾燥の少なくとも１つとして利用できるようにすれば、熱効率を一層高めることができるので好ましい。
【００２４】
【発明の実施の形態】
以下、本発明の一実施例を示す図面を参照しつつ、本発明をより具体的に説明していく。しかし、本発明はもとより下記図示例に限定されるわけではなく、前・後記の趣旨に適合し得る範囲で適当に変更を加えて実施することも勿論可能である。
【００２５】
図１は、本発明の一実施例を示す全体システムのフロー図であり、鉄源となる鉄鉱石(1)としては、好ましくは粒径８ｍｍ程度以下の粉鉱石を使用し、これを乾燥機(2)で乾燥したのち鉱石ミル(3)で粉砕する。乾燥機(2)の熱源としては、回転炉床炉(14)の排ガス顕熱を利用して予熱した空気(4)を使用し、必要により補助燃料(5)を使用する。炭素質還元剤としては、一般的には石炭(6)を使用し、石炭ミル(7)で粉砕してから混合機(8)へ送る。混合機(8)では、粉砕した粉鉱石と微粉炭、必要に応じておよびバインダー(9)や適量の水分を添加し、球状、粒状、ペレット状、ブリケット状などに塊成化し、成形体とする。このとき、溶解炉での還元溶融時に必要な副原料(10)（例えば、アルミナ、シリカ、カルシアなど）の一部、もしくは混合物をスラグ形成成分として添加することができる。
【００２６】
なお図示例では原料（混合物）を塊成化し成形体として用いる例を示しており、本発明ではこの様に成形して供給するのが最も好ましいので、以下の説明では成形体として使用する場合を主体にして説明するが、場合によっては粉状物を混合したままで使用し、あるいは軽く押し固めた程度の混合物として使用することも可能である。また鉄源としては、鉄鉱石が最も一般的であるが、これらと共に酸化鉄を含む高炉ダストやミルスケール等を併用してもよく、更には酸化鉄と共に非鉄金属やその酸化物を含むもの、例えば金属精錬設備から排出されるダストやスラグ等を使用することも可能である。
【００２７】
また、炭素質還元剤として石炭(6)などの炭材を使用する場合、炭材中に含まれる揮発分は６００℃以上の温度で揮発し酸化鉄の還元には殆ど寄与しないので、前記成形体中への炭材の配合率は、炭材中に揮発分として含まれる炭素を除いた炭素量を基準とし、該炭素量が酸化鉄の還元に必要な化学当量分と、溶鉄製品中の目標炭素濃度分、および溶解炉での還元鉄の溶解に必要な熱量分の総量に、若干のロスを加味して決めればよい。
【００２８】
原料成形体(12)を製造する際に用いる塊成機(11)としては、ペレタイザーやブリケッター等を使用できる。該成形体(12)は、見掛け密度で１．２ｇ／ｃｍ3以上、望ましくは１．８ｇ／ｃｍ3以上とすることが望ましい。これは、加熱還元炉（回転炉床炉）で成形体の外面側に与えられる熱が、成形体内部へ速やかに伝達するために必要な見掛け密度として見出した値である。
【００２９】
この成形体(12)は、成形後乾燥機(13)で水分量が１質量％程度以下となるまで乾燥してから回転炉床炉(14)（加熱還元炉）へ供給するのがよい。この時に用いる乾燥用空気として、回転炉床炉(14)から抜き出される排ガス顕熱との熱交換により予熱した空気(4)を使用すれば、排熱の有効利用が増進されるので好ましい。なお該乾燥用空気の温度は、急速加熱による水分の急激な蒸発によって成形体(12)が爆裂等を起こすことのないよう、２００℃程度以下に抑えることが望ましい。乾燥された成形体(12)は逐次回転炉床炉(14)へ装入し、加熱還元に供される。
【００３０】
加熱還元により生成する還元鉄(15)の金属化率は、少なくとも６０％以上、好ましくは８０％以上、より好ましくは、後記図２でも説明する如くスクラップの溶解熱量に近い９０％以上に高めておくことが望ましい。該加熱還元に用いる燃料としては、溶解炉(16)から抜き出される還元性ガスを用い、回転炉床炉(14)の側壁などに設けたバーナーで燃焼させることによって成形体(12)を加熱する。
【００３１】
該加熱還元工程で還元鉄(15)の金属化率を６０％以上、より好ましくは８０％以上、更に好ましくは９０％以上に高めるには、バーナー燃焼状態を安定に維持することが必要であり、そのためには、溶解炉(16)から抜き出される排ガスを一旦冷却し、除塵することによって排ガス中のダスト濃度を５ｇ／Ｎｍ3以下、望ましくは１ｇ／Ｎｍ3以下に低減しておくことが望ましい。尚、設備の立ち上げ時や回転炉床炉(14)の熱補償時などに備えて、天然ガスや微粉炭などを外部燃料(17)として供給可能にしておくことも有効である。
【００３２】
回転炉床炉(14)内では、後記式(2)、(4)で示す様な反応によって発生するＣＯガスを、前記予熱空気(4)と下記反応式(1)
ＣＯ ＋ 1/2Ｏ2 → ＣＯ2 ……(1)
で示す如く２次燃焼させ、この反応熱を成形体(12)の加熱還元用の熱として使用する。これらの反応で、排ガス中の酸素量が実質的にゼロになるまで完全燃焼させることができ、２次燃焼率１００％を達成できる。これは、回転炉床炉(14)で炭材が有している潜在的熱エネルギーを使い切ることを意味しており、高いエネルギー効率を得ることができる。
【００３３】
回転炉床炉(14)で得られる還元鉄(15)は、一旦系外へ取り出してから溶解炉(16)へ装入してもよいが、好ましくは、実質的に冷却することなく高温を保った状態で溶解炉(16)へ装入すれば、熱効率を高める上で有利である。また溶解炉(16)への装入法としては重力落下を利用し、炉の上方から連続装入すればよい。このとき、還元鉄(15)の加熱溶解に必要な熱源となる炭材(18)や、スラグ成分調整用の副原料(19)も、同様に溶解炉(16)の上方から投入する。この上方から投入する方法は、装入設備の保全を容易にする。
【００３４】
そして、該溶解炉(16)へ投入される酸素源(20)と炭材(18)を反応（燃焼）させることによって、還元鉄(15)中に残存する未還元の酸化鉄を還元すると共に、還元鉄を加熱・溶解し、好ましくは炭素含有量が２％以上、より好ましくは２．５〜４．５％の溶鉄を製造する。
【００３５】
この際、溶解炉(16)で発生するＣＯガスの２次燃焼率が４０％以下、より好ましくは２０％以上、４０％以下となる様に酸素源(20)と炭材(18)の供給量を制御し、２次燃焼熱の溶湯への着熱効率を６０％以上、より好ましくは７５％以上、更に好ましくは８０％以上に高める。なお、２次燃焼率を４０％以下に、また着熱効率を６０％以上（より好ましくは７５％以上）に定めた理由は、追って更に詳述する。
【００３６】
酸素源(20)としては、好ましくは酸素濃度が９０％以上の高純度酸素を使用し、これを溶解炉(16)の湯面上のスラグ層に向けて上吹き、横吹き、もしくは底吹きすることによりスラグ層を撹拌する。なお、酸素の吹込みを上吹き若しくは横吹き構造とすれば、吹込み用羽口のメンテナンスを容易にし、溶解炉(16)本体を傾動させる必要もなくなるため、溶解炉を固定式の簡素な構造にできるので有利である。
【００３７】
また、酸素濃度が９０％以上の高純度酸素を用いることで２次燃焼率の制御が容易になると共に、溶解炉(16)で発生する還元性ガスを回転炉床炉(14)へ供給する際のガスカロリーを適切なレベル、すなわち必要な理論燃焼温度を確保するのに必要かつ十分な条件制御も容易となる。
【００３８】
この際、底吹き撹拌用として鉄溶湯内へ窒素などの不活性ガス(21)を吹き込み、撹拌を強化することで還元鉄(15)の溶解を促進させることも有効である。
【００３９】
なお、溶解炉(16)に供給する炭材(18)の一部又は全部、および／もしくは、溶解炉(16)に供給する炭材(18)とは別の炭材を、成形体(12)とは別に回転炉床炉(14)へ直接供給することもできる。この炭材は、回転炉床炉(14)の炉床上に床敷材として供給したり、成形体(12)供給用の装置を用いて成形体(12)と同時に回転炉床炉(14)へ供給したり、成形体(12)を回転炉床炉(14)へ供給した後に供給することもきる。回転炉床炉(14)へ供給されるこの炭材は、床敷材として使用する場合は粉状のものが好ましいが、成形体(12)と同時に回転炉床炉(14)へ供給したり、成形体(12)を回転炉床炉(14)へ供給した後に供給する場合は、必ずしも粉状である必要はなく、塊状であっても構わない。このように炭材を回転炉床炉(14)へ供給すると、該炭材中の揮発分が揮発することで加熱原料としての機能も発揮し、外部燃料(17)の供給量も低減できるので好ましい。
【００４０】
上記において、別の炭材とは、溶解炉(16)へ供給される炭材(18)と装入する炉が異なる場合や、別の種類の炭材であってもよいことを意味するもので、例えば、溶解炉(16)に装入する炭材(18)がコークスである場合に、回転炉床炉(14)に装入する別の炭材として、上記コークスの原料となる石炭を使用する様な場合を想定したものである。従って、別の炭材といっても全く別の炭材を意味するものではない。
【００４１】
またこの炭材は、回転炉床炉(14)内で加熱されることによりチャー化した後、溶解に必要な炭材（燃料）として溶解炉(16)へ供給される。該炭材として石炭を用いた場合は、回転炉床炉(14)内でチャー化することによって石炭の揮発分は無くなり、予熱されたチャーとして溶解炉(16)へ供給されるので、溶解炉(16)へ炭材(18)として直接供給する場合に比べて、溶解炉(16)へ投入した時に発生する排ガス量が少なくなり、該排ガス設備を縮小できる他、余剰排ガス(26)の量も低減できるので好ましい。
【００４２】
上記炭材［炭材(18)についても同じ］としては、石炭の他、木材チップや廃プラスチック、古タイヤなど、更には、揮発分が含まれていないコークスや木炭、コークスブリーズ等も使用できる。
【００４３】
溶解炉(16)の側壁には、溶鉄(22)と溶融スラグ(23)を取り出すためのタップホールを設ける。該タップホールの開口設置高さは、撹拌ガス(21)が吹き抜けしない位置に設定するのがよい。また溶解炉(16)は密閉可能な構造とし、該溶解炉(16)から発生するガスの全量、もしくは一部を燃料源として前記回転炉床炉(14)へ供給し、有効利用できるようにするのがよい。溶解炉(16)からの発生ガスを回転炉床炉(14)へ送るに当たっては、図示する如くガスを一旦冷却し、除塵装置(24)を通して除塵することにより、ダスト含有量を５ｇ／Ｎｍ3程度以下、望ましくは１ｇ／Ｎｍ3以下に低減しておくのがよい。それにより、ガス配管や回転炉床炉(14)などの内壁へのダストの付着堆積を可及的に抑えることができる。この際、該溶解炉(16)から排出される高温ガスが保有する顕熱を、例えば溶解炉(16)の出口に設けた輻射伝熱ボイラー等によって回収してからガス冷却・除塵装置(24)へ送る構成とすれば、排ガスの顕熱も有効に活用できるので好ましい。
【００４４】
その後、昇圧ブロワー(25)で圧力調整した後、回転炉床炉(14)の燃焼バーナーへ供給する。このとき、溶解炉(16)から抜き出される排ガスの燃料ガスとしての量が過剰である場合は、余剰排ガス(26)として外部へ取り出し、隣接設備の燃料ガスとして有効利用すればよい。なお、溶解炉(16)を密閉構造とし且つ高圧の酸素ガスを使用すれば、その圧力を利用して溶解炉(16)内を加圧することができ、昇圧ブロワー(25)を省くことができる。
【００４５】
回転炉床炉(14)から排出されるガスは、潜熱を殆ど有していないものの高温であるから、廃熱ボイラー(27)で熱回収した後、空気予熱用熱交換器(28)で空気の予熱に有効利用すればよい。熱交換器(28)で熱回収された排ガスは、除塵装置(30)で浄化処理した後、吸引ファン(31)を経て大気放散される。この吸引ファンにより回転炉床炉(14)の炉内圧力が制御される。
【００４６】
本発明は、上記の様なフロー図に沿って実施されるが、その中でも特に重要となる回転炉床炉(14)と溶解炉(16)の操業条件などについて、更に詳しく説明する。
【００４７】
まず、還元鉄製造設備の主体となる回転炉床炉について詳述する。酸化鉄含有物質と炭素質還元剤を含む混合物、好ましくはこれらを成形してなる炭材内装成形体を回転炉床炉へ供給して加熱すると、下記式(2)〜(4)
FemOn ＋ nC → mFe ＋ nCO ……(2)
FemOn ＋ nCO → mFe ＋ nCO2 ……(3)
C ＋ CO2 → ２CO……(4)
で示される反応が進行し、酸化鉄の還元が行われる。ここで発生するＣＯやＣＯ2の量は、成形体中に内装された炭素質還元剤の量や加熱条件により決ってくる。
【００４８】
回転炉床上に装入された原料混合物は、バーナー燃焼による燃焼熱と炉壁および天井からの輻射伝熱によって加熱される。熱輻射は温度の４乗で作用するため、迅速な昇温と還元が可能であり、原料混合物中の酸化鉄は例えば６〜１２分といった極短時間の加熱で金属鉄に還元される。
【００４９】
原料混合物の外面側から与えられる熱は、伝導伝熱で原料混合物の内部へ伝わり、前記式(2)〜(4)の反応を継続させる。該混合物内部方向への伝熱を効率よく進めるには、原料混合物を成形体とし、その見掛け密度を１．２ｇ／ｃｍ3以上、望ましくは１．８ｇ／ｃｍ3以上にしておくことが望ましい。
【００５０】
酸化鉄源と炭素質還元剤の混合比は、炭素質還元剤中の揮発分を除いた固定炭素分が酸化鉄の還元に要する化学当量以上となる様にすべきことは当然であり、更には、溶解炉へ投入した後の加熱溶融に必要な燃焼熱量と、還元溶融によって生成する溶鉄の目標炭素濃度相当量も加味して決めるのがよい。
【００５１】
即ち炭素質還元剤および前記炭材の量は、当該炭素質還元剤および前記炭材中の揮発分を除いた炭素量（Ａ）が、［（当該混合物中の酸化鉄の還元に必要な化学当量）＋（溶鉄製品中の目標炭素濃度分）＋（固形還元鉄の溶解に必要な熱量分）］以上となる様に調整すべきであり、該炭素量（Ａ）の調整は、加熱還元炉へ装入される前記混合物中へ配合する炭素質還元剤、加熱還元炉で製造された還元鉄中へ配合する炭素質還元剤および前記溶解炉へ供給される炭材から選ばれるの１つ以上の量によって行えばよい。例えば、原料混合物の調製段階で多量の炭材を内装した場合は、それに応じて、加熱還元で得た固形還元鉄へ混入する炭材の量を適宜減少させればよい。
【００５２】
また溶解炉で還元溶融を行う際には、固形還元鉄と共に或いは別途、溶解炉にＣａＯ含有物質を添加し、副生スラグの塩基度が好ましくは１．２以上となる様に調整することが好ましい。ちなみに、溶解炉で副生するスラグの塩基度を１．２以上に調整してやれば、鉄溶湯中に含まれている硫黄分が溶融スラグ方向へ移行し、得られる金属鉄の硫黄含量を低減できるので好ましい。
【００５３】
この時、副生スラグ中のＦｅＯ含量が少なくなるにつれて、硫黄成分のスラグ方向への分配率が高まり、鉄溶湯中の硫黄含量は減少する。スラグ中のＦｅＯ含量は鉄溶湯中の炭素量（Ｂ）が多くなるほど減少してくるので、スラグ方向への硫黄成分の分配率を高めて鉄溶湯中の硫黄含量を低減させるには、鉄溶湯中の炭素量（Ｂ）を２％程度以上、より好ましくは３％程度以上に高めることが有効となる。この様にしてスラグ中のＦｅＯ量を少なくすれば、溶融ＦｅＯによる炉内張り耐火物の溶損も抑えられるので好ましい。
【００５４】
該鉄溶湯中の炭素量（Ｂ）は、
▲１▼加熱還元炉へ装入される前記原料混合物内へ配合する炭素質還元剤、
▲２▼加熱還元炉で製造された還元鉄中へ配合する炭素質還元剤、
▲３▼前記溶解炉へ供給される炭材、
▲４▼前記加熱還元炉へ装入される別の炭材
の何れか１以上によって行えばよい。
【００５５】
ところで、還元鉄の還元溶融が行われる溶解炉の特性としては、溶解炉での還元溶融処理を効率よく進めるため、溶解炉へ装入される鉄源（還元鉄）の金属化率を如何に高めておくかということが鍵であり、そのためには、回転炉床炉において還元鉄の金属化率を如何に高めるかが重要となる。
【００５６】
そのためには、回転炉床炉における原料成形体の昇温・加熱条件を適切且つ安定に制御しなければならず、加熱用燃料ガスの性状を極力安定に維持すべきである。前記溶解炉で発生するガスを回転炉床炉へ送り燃料ガスとして使用する際に、該ガスのカロリーが高いほど燃焼温度を高め易く、回転炉床炉の温度制御が容易となる。これは、溶解炉での２次燃焼率を低めに抑え、ＣＯ2含量を低めに維持することが好ましいことを意味している。また、バーナー燃焼を安定して長時間継続させるには、燃料ガス中のダストを可能な限り少なく抑え、送給配管内や燃料ガスバーナへのダストの付着堆積やノズル詰り等を可及的に防止することが望ましい。
【００５７】
そこで、溶解炉からの排出ガスを回転炉床炉へ導くまでの間で、該ガスを一旦冷却して除塵する設備を設ける。この除塵処理によって、ガス中のダスト濃度を５ｇ／Ｎｍ3以下、望ましくは１ｇ／Ｎｍ3以下に低減させることが推奨される。なお除塵設備の操業温度は、設備の耐熱性や安全性などを考慮して８００℃程度以下に抑えるのがよい。
【００５８】
次に、固形還元鉄の還元溶融が行われる溶解炉の操業条件について説明する。
【００５９】
溶解炉内の鉄浴に投入される炭材は、同時に吹き込まれる高濃度酸素との反応によって下記式(5)で示される如くＣＯガスを発生し
Ｃ ＋ 1/2Ｏ2 → ＣＯ ……(5)
鉄浴上の気相内で下記式(6)で示す如く２次燃焼を起こす。
【００６０】
ＣＯ ＋ 1/2Ｏ2 →ＣＯ2 ……(6)
これらの反応は発熱反応であり、これらの熱が鉄浴へ伝えられ、溶解炉へ投入される固形還元鉄を更に還元し溶融するための熱として使用される。
【００６１】
第２，３図は、溶解炉へ投入される鉄源の金属化率と、溶解炉における２次燃焼率および炭材消費量の関係を示したグラフである。これらの図からも明らかな様に、炭材消費量は、投入される鉄源の金属化率の上昇と共に減少し（図２）、また２次燃焼率の上昇と共に減少する（図３）。
【００６２】
特に図２によれば、２次燃焼率が４０％以下では、金属化率が６０％以上になると炭材消費量は横這い状態となり、金属化率の変動による炭材消費量の変動が少なくなるため、安定操業を遂行する上で極めて有用となる。
【００６３】
よって、溶解炉へ供給する鉄源（即ち還元鉄）の金属化率は、炭材消費量を抑えると共に安定操業を増進する上で極力高める方が有利であり、少なくとも６０％以上、より好ましくは８０％以上、更に好ましくは通常の鉄スクラップに相当する９０％以上とすることが望ましい。
【００６４】
なお金属化率で６０％以上を確保するには、原料混合物の製造時に配合される炭素質還元剤の量や回転炉床炉での加熱還元条件を適切に制御すればよく、具体的には、原料混合物の調製段階で前述した如く酸化鉄の還元に必要十分量の炭素質還元剤を配合する他、回転炉床炉での操業温度を1100〜1400℃、より好ましくは1250〜1350℃とし、滞留時間で６分以上、より好ましくは８分以上を確保すればよい。
【００６５】
なお図３からは、溶解炉での２次燃焼率の上昇による炭材消費量低減効果を実操業において有効に発揮させるには、２次燃焼率を高めにすることが望ましく、より好ましくは２０％以上を確保するのがよい。しかし２次燃焼率が４０％を超えると、それ以上の炭材消費量低減効果は殆ど認められなくなるので、２次燃焼率は４０％以下、より好ましくは３０％程度以下に抑えるのがよい。
【００６６】
なお２次燃焼率は、溶解炉への炭材の添加量や酸素ガス吹き込み量などによって変わってくるので、その値を４０％以下、より好ましくは２０〜４０％の範囲に制御するには、前記２次燃焼率を考慮して、炭材添加量や酸素ガス吹き込み量を適切に制御すればよい。
【００６７】
また２次燃焼は、溶解炉における気相側の温度を上昇させ、内張り耐火物に多大な熱負荷を与える。投入鉄源の金属化率が低下するということは、鉄源中に含まれる未還元酸化鉄（ＦｅＯ）量が多くなることを意味しており、延いては副生スラグ中のＦｅＯ量の増大によって内張り耐火物の溶損を加速する。そこで、前述の如く耐火物の溶損を抑えるための手段として耐火物を水冷することも試みられているが、この方法では水冷による熱損失が生産効率やコストに重大な影響を及ぼす。
【００６８】
また、溶解炉へ添加される鉄源（還元鉄）の溶解を促進するには、鉄浴を撹拌することが有効であるが、撹拌を強化すると溶解炉から抜出される排ガス中のダスト量が増大し（最大１００ｇ／Ｎｍ3程度まで）、鉄の歩留りを低下させるばかりでなく、高温ガス配管内に付着・堆積して閉塞事故を引き起こす原因になる。
【００６９】
これらのことを考慮して本発明では、溶解炉へ供給する還元鉄の金属化率を６０％以上、より好ましくは８０％以上に高めることで炭材消費量を低減し、且つ、溶解炉での２次燃焼率を４０％以下、より好ましくは２０〜４０％程度、更に好ましくは２０〜３５％に抑制することによって、気相側温度の過度の上昇を回避し、溶解炉への負荷を軽減できるようにしている。
【００７０】
尚、溶解炉へ吹き込む酸素源としては空気を用いることもできるが、その場合は酸素に対して約４倍量も含まれる窒素も予熱されることになり、予熱エネルギーの無駄が多くなるばかりでなく、発生ガス量も増大してくる。従って、熱効率を高めると共に無為なガス発生量の増大を避けるには、酸素源として高純度酸素、好ましくは酸素純度が９０％以上の高純度酸素を使用することが望ましく、それにより、発生ガス量を最小限に抑えると共に、ダスト発生量も低減できる。
【００７１】
次に、溶解炉における２次燃焼率と着熱効率、および溶解炉からの排出ガス温度の関係を調査し、従来例と比較検討した結果を図４に示す。
【００７２】
図４からも明らかな様に、着熱効率が一定であれば、２次燃焼率が高まるにつれて排ガス温度は上昇し、溶解炉で有効に使われることなく排出される熱量が増大する。逆に、排ガス温度を一定に保つことができれば、２次燃焼率が高まるにつれて着熱効率は上昇し、熱が有効に使われることを確認できる。図４に示した事例Ａは、溶解炉への投入鉄源としてスクラップを用いた場合の例であり、２次燃焼率が２０％で着熱効率は８９％と高く、排ガス温度も1650℃程度の低い結果が得られている。
【００７３】
これに対し事例Ｂは、溶解炉への投入鉄源として金属化率が３０％の還元鉄を用いた場合で、２次燃焼率は約４５％に高まっているため、排ガス温度は1900℃の高温となって内張り耐火物への熱負荷が増大する他、着熱効率は８５％に低下している。更にこの事例Ｂでは、投入鉄源の金属化率が３０％と低いため、還元溶融時に副生するスラグ中の（ＦｅＯ）濃度が高くなり、内張耐火物の溶損も加速されることが確認された。
【００７４】
これらの結果から、回転炉床炉による加熱還元装置と、生成する還元鉄を還元溶融する溶解炉を連結した一貫設備を操業する際の望ましい条件としては、
▲１▼回転炉床炉での金属化率を６０％以上、より好ましくは８０％以上に高めて残留（ＦｅＯ）を極力少なくすること、
▲２▼溶解炉からの排出ガスを回転炉床炉の燃料に使用するための必要なカロリーを確保するため、溶解炉での２次燃焼率を４０％以下、より好ましくは２０〜４０％の範囲に制御すること、
▲３▼溶解炉の内張り耐火物の溶損を抑えるため、排ガス温度を過度に高めないためにも、２次燃焼率は４０％以下に抑えること
が重要であり、図４の斜線で示した領域が好ましい条件として推奨される。
【００７５】
即ち、先の第２，３図で説明した如く、回転炉床炉での加熱還元後溶解炉へ装入される還元鉄の金属化率を６０％以上に高め、且つ、溶解炉で発生するＣＯガスの２次燃焼率が４０％以下となる様に、該溶解炉への酸素および炭材の供給量を制御し、２次燃焼熱の鉄溶湯への着熱効率を６０％以上、より好ましくは７５％以上に高めればよいことが確認された。
【００７６】
なお、２次燃焼熱の鉄溶湯への着熱効率（Ｅｆ）は、次の様に定義される。
【００７７】
Ｅｆ＝［１-(Ｈ3＋Ｈ4−Ｈ2)／Ｈ1］×１００（％）
Ｈ1：２次燃焼反応の発熱量。ここで、２次燃焼反応とは、鉄溶湯から発生するＣＯ、Ｈ2ガスの酸素による燃焼で、下記反応式で示される。
【００７８】
ＣＯ＋（1/2）Ｏ2＝ＣＯ2
Ｈ2＋（1/2）Ｏ2＝Ｈ2Ｏ
Ｈ2：鉄溶湯から発生するガスの顕熱。ガス量と組成は鉄溶湯の物質収支から算出され、温度は鉄溶湯と同じとする。
【００７９】
Ｈ3：炉から排出されるガスの顕熱。
【００８０】
Ｈ4：２次燃焼反応が生じている気相側からの熱損失（全入熱量の１０〜２０％に相当する）。
【００８１】
こうした条件を満たせば、溶解炉の内張り耐火物の寿命が延長するので、溶解炉を中間補修やメンテナンスのために傾動可能にしたり移動可能にする必要がなく、固定型の溶解炉本体を用いた場合でも長期間支障なく操業を続けることが可能となる。但し、本発明では固定型溶解炉の使用に限定されるわけではなく、転動式の溶解炉を使用することも勿論可能である。
【００８２】
かくして本発明によれば、炭素質還元剤が内装された原料混合物を回転炉床炉の如き加熱還元炉へ装入し、該混合物中の酸化鉄を還元して固形還元鉄とした後、これを溶解炉へ送って更に加熱還元すると共に還元鉄を溶解して溶鉄を製造する際に、
a)加熱還元炉で固形還元鉄の金属化率を６０％以上に進め、
b)溶解炉で発生するＣＯガスの２次燃焼率が４０％以下となるように酸素供給量と炭材供給量を制御し、
c)好ましくは、該２次燃焼による燃焼熱の溶湯への着熱効率を６０％以上に高め、
d)溶解炉を密閉構造として、該溶解炉から発生するガスの全部もしくは一部を燃料として前記加熱還元炉へ供給し、得られる固形還元鉄を、前記溶解炉で加熱することにより、
炭素含有量が１．５〜４．５％程度の還元鉄溶湯を高いエネルギー効率の下で、加熱還元炉や溶解炉の劣化を最小限に抑制しつつ生産性よく製造し得ることになった。
【００８３】
【実施例】
以下、実施例を挙げて本発明をより具体的に説明するが、本発明はもとより下記実施例によって制限を受けるものではなく、前・後記の趣旨に適合し得る範囲で適当に変更を加えて実施することも可能であり、それらはいずれも本発明の技術的範囲に含まれる。
【００８４】
実施例
前記図１として示したプロセスフロー図に準拠し、表１に示す化学組成の原料鉱石と石炭を用いて、表２に示す条件で試験操業を行い、表２に併記する結果を得た。
【００８５】
【表１】
【表２】
表２において、Ｎｏ．１〜３は、回転炉床炉で製造される還元鉄の金属化率を９０％に保ち、溶解炉では２次燃焼率を４０％以下に、着熱効率が６０〜９０％になるように制御したもので、このうちＮｏ．１は、溶解炉発生ガスを全量回転炉床炉へ導入し、熱量不足分を補助燃料（本例では天然ガスを使用）で補ったケースである。
【００８８】
Ｎｏ．２は、溶解炉の着熱効率を高めると共にガス発生量を増やし、回転炉床炉での補助燃料ゼロを目指した例である。その結果、溶解炉からの排ガス中のダスト量は若干増大するが、実操業の障害となるまでには至らない。また、溶解炉からの排ガス量が若干余剰気味となり、僅かではあるが、外部熱源として利用できることが分る。
【００８９】
Ｎｏ．３は、全プロセスパラメータを最適化し、回転炉床炉での補助燃料を使用せず、同時に溶解炉からの余剰ガスもゼロを目指した例であり、回転炉床炉と溶解炉を結び、エネルギー的に自己完結型の操業が確立されている。
【００９０】
これらに対しＮｏ．４では、２次燃焼効率が３０％と低めに保たれているが、溶解炉での鉄溶湯への着熱効率が７３％とやや低いため、石炭および酸素の使用量がやや増大し、余剰ガスも発生量やダスト濃度もやや増加する傾向が見られる。Ｎｏ．６は溶解炉への炭材装入量を増やして浸炭量を増大し、溶鉄の炭素量を飽和炭素量レベルにまで高めた例である。即ち本発明によれば、溶解炉への吹込み炭素量などを調整することにより、溶鉄の炭素量を飽和炭素濃度レベルまでに高めることの容易である。
【００９１】
Ｎｏ．５では、溶解炉での２次燃焼率を過度に高めた例であり、着熱効率は高められているが、加熱還元炉へ送給される排ガス量と熱量（還元ポテンシャル）が低下したため、回転炉床炉では補助燃料による追い炊きが必要となった。
【００９２】
これらの結果からも明らかな様に、前掲の操業条件を最適化すれば、高いエネルギー効率の下で溶解炉への過度の熱負荷を与えることなく、固体還元から還元溶融にわたる一連の操業を安定して効率よく実施し、高純度の溶融鉄を生産性よく製造できる。そして、例えば上記Ｎｏ．３で示した如く、操業条件をうまくコントロールすれば、一連の溶鉄生産設備内でエネルギー的に自己完結型の操業も実現可能となる。
【００９３】
尚、上記Ｎｏ．３と同様の条件で溶融金属鉄の製造を行う際に、溶解炉に加熱用の炭材と共に生石灰（ＣａＯ）を追加投入することによって、生成スラグの塩基度（ＣａＯ／ＳｉＯ2比）が１．５〜１．６の範囲となる様に調整しながら還元溶融を継続し、得られる溶鉄のＳ含量を測定した。その結果、操業開始の初期段階にはＳ含量が徐々に増大し、４０分経過するとＳ含量は約0.04質量％にまで高まったが、その後のＳ含量の増大はみられず、得られる金属鉄のＳ含量は約0.04質量％で安定していた。これは、溶解炉に生石灰を追加投入して生成スラグの塩基度を高めることで、溶融金属中のＳがスラグ方向へ移行したことによるものと考えられる。
【００９４】
【発明の効果】
以上説明した様に本発明によれば、従来法に比べてより少ないエネルギー消費量で溶鉄を効率よく製造することができ、しかも耐火物損耗が少なくてエネルギー柔軟性に富み、生産弾力性のある製鉄プロセスを提供できる。
【図面の簡単な説明】
【図１】本発明の一実施例を示すプロセスフロー図である。
【図２】溶解炉での２次燃焼率を変えたときの、溶解炉へ供給される鉄分の金属化率と炭材消費量の関係を示すグラフである。
【図３】溶解炉へ投入される鉄分の金属化率を変えたときの炭材消費量と２次燃焼率の関係を示すグラフである。
【図４】溶解炉からの排出ガス温度を変えたときの、溶解炉内鉄浴への着熱効率と２次燃焼率の関係を示すグラフである。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for producing molten iron, and more specifically, an iron oxide source such as iron ore is heat-reduced together with a carbonaceous reducing agent such as a carbonaceous material, so that molten iron having a high iron purity can be efficiently produced. It is about the method.
[0002]
[Prior art]
The blast furnace-converter method is currently in practical use as a method for producing molten iron by reducing an iron oxide source such as iron ore, but this method requires the use of coke as a reducing agent. In addition, there is a lack of production flexibility in response to economic trends because of the pursuit of economies of scale, and there is a problem particularly in dealing with high-mix low-volume production.
[0003]
On the other hand, there is a direct ironmaking method represented by the MIDREX method as a small-scale ironmaking method suitable for high-mix, low-volume production. However, since this method requires natural gas as a reducing agent, there are restrictions on the location conditions of the equipment.
[0004]
On the other hand, there is an SL / RN method as a method for producing reduced iron by producing reduced iron using a coal-based carbonaceous reducing agent and heating and melting the reduced iron in an electric furnace. A number of direct iron making methods have also been proposed in which a furnace and an electric melting furnace are combined and the reduction of iron oxide and the heating and melting of the resulting reduced iron are performed consistently. However, since these methods require a large amount of electric power, the location conditions of the equipment are limited to places where power supply conditions are good.
[0005]
Under the above-mentioned situation, research on improvement of the melting / reducing iron making method for producing molten iron using an iron source such as iron ore and a carbonaceous reducing agent such as coal has been actively pursued. The DIOS method and the HIsme1t method combining a pre-reduction furnace and a smelting reduction furnace have been proposed. What is important in putting these methods to practical use is to ensure a high level of secondary combustion rate and heat-emission efficiency in a smelting reduction furnace. The problem that high-concentration iron oxide (FeO) is mixed in the slag which is produced as a byproduct during heating and reduction due to the gangue component, and remarkably dissolves the refractory lining of the processing furnace, has been highlighted. As a countermeasure against this problem, a method has been proposed in which the furnace body is water-cooled to suppress the erosion of the refractory.However, the heat loss from the furnace body increases, so that the productivity and heat energy efficiency of molten iron are greatly reduced. Adversely affect.
[0006]
In addition, as one of the direct iron making methods, a carbonaceous interior molded article (pellet, briquette, etc.) formed by mixing an iron source such as iron ore and a carbonaceous reducing agent such as carbonaceous material is heated and reduced in a rotary hearth furnace. A method of producing molten iron by finally performing smelting reduction in a smelting reduction furnace is known (
[0007]
In addition, when heat fluctuation occurs in the smelting reduction furnace, the calorific value and the reduction potential of the high-temperature gas supplied to the rotary hearth furnace fluctuate, and the operation state of the entire equipment becomes unstable. When the operating conditions become unstable, the reduction efficiency and metallization rate of the iron oxide in the rotary hearth furnace fluctuate, which not only makes the purity of the product iron unstable, but also causes the iron oxide to enter the by-product slag. The amount of (FeO) mixed increases, which causes the hearth refractory to melt.
[0008]
Furthermore, in the smelting reduction method, a large amount of oxygen and heat are added to the smelting reduction furnace, so repair of the furnace body refractory and maintenance of the blowing tuyere are indispensable. Equipment is required, and the economic burden of installing these additional equipment and repairing refractories has a considerable effect on the production cost of molten iron.
[0009]
[Patent Document 1]
Japanese Patent Publication No. 3-60883 (Claims, Fig. 1, etc.)
[Patent Document 2]
JP-A-2001-279313 (Claims, FIG. 3, etc.)
[Patent Document 3]
JP-A-2001-247920 (claims, FIG. 1 and the like).
[0010]
[Problems to be solved by the invention]
The present invention has been made in view of the situation as described above, and its object is to use a mixture containing iron oxide and a carbonaceous reducing agent as a raw material and to provide a molten iron manufacturing process combining a rotary hearth furnace and a melting furnace. In these, by appropriately controlling the operating conditions, it is possible to produce molten iron with high iron purity with high productivity while suppressing erosion of refractories of a rotary hearth furnace and a melting furnace as much as possible. It is to provide a method.
[0011]
[Means for Solving the Problems]
The method for producing molten iron according to the present invention, which can achieve the above object, is that a raw material mixture containing an iron oxide source and a carbonaceous reducing agent is charged into a heating reduction furnace, and iron oxide in the mixture is converted into a carbonaceous reducing agent. After reducing to solid reduced iron, the solid reduced iron is sent to a melting furnace, and the carbon material supplied as fuel is burned, thereby melting the solid reduced iron in the melting furnace to obtain a molten iron. In the process of producing molten iron,
The secondary combustion of CO gas in the melting furnace is performed by increasing the metallization rate of the solid reduced iron to 60% or more and then sending it to the melting furnace and controlling the amounts of oxygen and carbon material supplied to the melting furnace. The point lies in controlling the rate to 40% or less.
[0012]
In the present invention, the secondary combustion rate in the melting furnace is a value calculated by the following equation from an analysis value of a gas component obtained by continuously sampling the exhaust gas from the melting furnace,
Secondary combustion rate = 100 x (COTwo+ HTwoO) / (CO + COTwo+ HTwo+ HTwoO)
The heat transfer efficiency is calculated using the measured values of the temperature of the exhaust gas from the melting furnace and the temperature of the molten iron, and the secondary combustion rate obtained by the above equation.
[0013]
In preparing the raw material mixture used in the present invention, the amount of the carbonaceous reducing agent and the amount of the carbonaceous material is adjusted by the amount of carbon (A) excluding the volatile matter in the carbonaceous reducing agent and the carbonaceous material. , [(Chemical equivalent required for reducing iron oxide in the mixture) + (target carbon concentration in molten iron product) + (caloric value required for dissolving solid reduced iron)] This is preferable because a series of steps from solid reduction to reduction melting of iron oxide in the raw material mixture to obtaining molten metal iron can be performed more efficiently and smoothly.
[0014]
The carbon amount (A) is adjusted by adjusting the carbonaceous reducing agent to be blended into the mixture charged into the heating reduction furnace, the carbonaceous reducing agent to be blended into reduced iron produced by the heating reduction furnace, and It can be carried out by one or more selected from carbonaceous materials supplied to the furnace.
[0015]
When high-purity oxygen having an oxygen concentration of 90% or more is used as the oxygen-containing gas to be supplied to the melting furnace, not only the secondary combustion efficiency in the melting furnace can be increased, but also the combustion temperature during the secondary combustion can be improved. This is preferable because the control of the heat transfer efficiency to the molten iron bath or the molten iron bath is further facilitated, and the amount of generated gas can be reduced to reduce the amount of dust generated. The supply of the high oxygen concentration gas to the melting furnace can be performed by bottom blowing, top blowing, side blowing, or any combination thereof. Among them, the high oxygen concentration gas is supplied to the slag layer. Blowing upward or sideward is preferable because the added carbonaceous material can be efficiently burned in the slag layer, and the heating efficiency can be increased.
[0016]
The melting furnace is a fixed type or a rolling type, and the flux for adjusting the solid reduced iron, the carbonaceous material, and the slag component to the melting furnace is supplied by gravity drop from above the melting furnace into the furnace, or It is preferable to employ a method in which the molten metal is blown into the molten metal, because reduction melting can be efficiently advanced by a simple operation. At this time, it is preferable to blow an inert gas into the molten iron of the melting furnace and stir the molten iron, since the melting of the solid reduced iron is further accelerated and the processing time can be shortened.
[0017]
When using a fixed melting furnace, if the tap hole for removing molten iron and molten slag provided on the side wall of the melting furnace is set at a position where the opening height position does not cause blow-through of inert gas, This is preferable because it is possible to prevent an obstruction of the blowing tuyere due to gas blowing.
[0018]
The iron oxide source used in the present invention is most commonly iron ore, but it is also possible to use a mill scale or the like, and it is also possible to use an oxidized material such as blast furnace dust or converter dust. Iron-containing dust, and those containing non-ferrous metals and their oxides along with iron oxides, such as nickel, chromium, manganese, ores containing non-ferrous metals and their oxides, such as titanium, and discharged from metal refining facilities It is also possible to use dust or slag.
[0019]
In the case of the above-mentioned non-ferrous metal or its oxide, it can be transferred to slag generated in the process of producing molten iron, and can be recovered as a raw material or a product of high-purity non-ferrous metal or its oxide.
[0020]
When dissolving the solid reduced iron in the melting furnace, a considerable amount of sulfur is mixed into the molten metal iron derived from the carbonaceous material and the like, but an appropriate amount of a CaO-containing substance is added in this melting step, The basicity of the slag generated in the melting furnace (CaO / SiOTwo) Is adjusted to be 1.2 or more, since the sulfur content in the molten metallic iron can be shifted in the direction of the molten slag, and the sulfur content of the metallic iron can be reduced, which is preferable. At this time, if the amount of the carbon material added to the melting furnace is adjusted so that the carbon content of the molten metal iron becomes 2% or more, the distribution ratio of sulfur in the slag direction is increased, and the sulfur content in the molten iron is increased. Is preferred because it can be reduced more efficiently.
[0021]
Further, it is preferable to increase the efficiency of the secondary combustion heat to heat the molten iron to 60% or more.
[0022]
Solid reduced iron obtained in a heating and reducing furnace is preferable because it can be effectively used for heating and melting by holding the solid reduced iron directly into the melting furnace while maintaining a high temperature. It is, of course, possible to temporarily store the solid reduced iron in a yard or the like and send it to a melting furnace as needed to perform heating and melting.
[0023]
When the above method is carried out, the combustion gas generated in the melting furnace has a considerable amount of heat, so it can be sent to a heating reduction furnace and effectively used as a heat source. In order to avoid the problem of dust adhesion to the supply pipe and the heating reduction furnace, the combustion gas is cooled and dust-removed, and the dust content in the gas is reduced to 5 g / Nm.ThreeIt is preferable to suppress the following, and also to preheat air by using the exhaust gas from the heating and reducing furnace, to dry the combustion air in the heating and reducing furnace, the raw material mixture, and furthermore, to use the iron oxide source as the raw material of the raw material mixture. And at least one of the drying of the carbonaceous reducing agent is preferable because the thermal efficiency can be further improved.
[0024]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described more specifically with reference to the drawings illustrating an embodiment of the present invention. However, the present invention is not necessarily limited to the illustrated examples below, and it is needless to say that the present invention can be embodied with appropriate modifications within a range that can be adapted to the spirit of the preceding and the following.
[0025]
FIG. 1 is a flow chart of an entire system showing one embodiment of the present invention. As an iron ore (1) serving as an iron source, preferably, a fine ore having a particle size of about 8 mm or less is used, and this is dried. After drying in (2), it is ground in an ore mill (3). As a heat source of the dryer (2), air (4) preheated by utilizing sensible heat of exhaust gas from a rotary hearth furnace (14) is used, and an auxiliary fuel (5) is used as necessary. Generally, coal (6) is used as a carbonaceous reducing agent, and is pulverized by a coal mill (7) and then sent to a mixer (8). In the mixing machine (8), pulverized fine ore and pulverized coal, if necessary, and a binder (9) and an appropriate amount of water are added, and agglomerated into spherical, granular, pellet, briquette, etc. I do. At this time, a part or mixture of the auxiliary raw materials (10) (for example, alumina, silica, calcia, etc.) required for reduction melting in a melting furnace can be added as a slag forming component.
[0026]
In the illustrated example, an example is shown in which the raw material (mixture) is agglomerated and used as a molded body. In the present invention, it is most preferable that the raw material (mixture) is molded and supplied as described above. Although mainly described, depending on the case, it is also possible to use the powdery substance as it is mixed, or to use it as a lightly compacted mixture. As the iron source, iron ore is the most common, but blast furnace dust or mill scale containing iron oxide may be used in combination therewith. For example, dust or slag discharged from a metal refining facility can be used.
[0027]
When a carbonaceous material such as coal (6) is used as the carbonaceous reducing agent, the volatile matter contained in the carbonaceous material volatilizes at a temperature of 600 ° C. or more and hardly contributes to the reduction of iron oxide. The blending ratio of the carbon material in the body is based on the amount of carbon excluding carbon contained as volatile matter in the carbon material, and the amount of carbon is the chemical equivalent required for the reduction of iron oxide, and the amount of carbon in the molten iron product. It may be determined by adding a slight loss to the target carbon concentration and the total amount of heat required for melting the reduced iron in the melting furnace.
[0028]
A pelletizer, a briquetter, or the like can be used as the agglomerator (11) used when producing the raw material molded body (12). The molded body (12) has an apparent density of 1.2 g / cm.ThreeAbove, desirably 1.8 g / cmThreeIt is desirable to make the above. This is a value found as an apparent density necessary for the heat given to the outer surface side of the compact in the heating reduction furnace (rotary hearth furnace) to be quickly transmitted to the inside of the compact.
[0029]
The molded body (12) is preferably dried by a dryer (13) after molding until the water content becomes about 1% by mass or less, and then supplied to the rotary hearth furnace (14) (heating reduction furnace). It is preferable to use air (4) preheated by heat exchange with exhaust gas sensible heat extracted from the rotary hearth furnace (14) as the drying air used at this time, because the effective use of exhaust heat is enhanced. The temperature of the drying air is desirably suppressed to about 200 ° C. or less so that the molded body (12) does not explode due to rapid evaporation of water due to rapid heating. The dried compacts (12) are sequentially charged into a rotary hearth furnace (14) and subjected to heat reduction.
[0030]
The metallization ratio of the reduced iron (15) generated by heat reduction is increased to at least 60% or more, preferably 80% or more, and more preferably to 90% or more, which is close to the heat of dissolution of scrap as also described in FIG. It is desirable to keep. As the fuel used for the heat reduction, a reducing gas extracted from the melting furnace (16) is used, and the compact (12) is heated by burning it with a burner provided on the side wall of the rotary hearth furnace (14). I do.
[0031]
In order to increase the metallization ratio of the reduced iron (15) to 60% or more, more preferably 80% or more, and still more preferably 90% or more in the heat reduction step, it is necessary to stably maintain the burner combustion state. For this purpose, the exhaust gas extracted from the melting furnace (16) is once cooled and dust-removed to reduce the dust concentration in the exhaust gas to 5 g / Nm.ThreeBelow, desirably 1 g / NmThreeIt is desirable to reduce it below. It is also effective to be able to supply natural gas, pulverized coal and the like as the external fuel (17) in preparation for starting up the facilities and for heat compensation of the rotary hearth furnace (14).
[0032]
In the rotary hearth furnace (14), CO gas generated by a reaction represented by the following formulas (2) and (4) is mixed with the preheated air (4) and the following reaction formula (1).
CO + 1 / 2OTwo → COTwo …… (1)
The reaction heat is used as heat for heat reduction of the molded body (12). By these reactions, complete combustion can be performed until the amount of oxygen in the exhaust gas becomes substantially zero, and a secondary combustion rate of 100% can be achieved. This means that the potential heat energy of the carbon material in the rotary hearth furnace (14) is used up, and high energy efficiency can be obtained.
[0033]
The reduced iron (15) obtained in the rotary hearth furnace (14) may be once taken out of the system and then charged into the melting furnace (16). If it is charged into the melting furnace (16) while maintaining it, it is advantageous in increasing the thermal efficiency. As a method for charging the melting furnace (16), gravity drop may be used, and the melting furnace (16) may be continuously charged from above the furnace. At this time, a carbon material (18) serving as a heat source necessary for heating and melting the reduced iron (15) and an auxiliary material (19) for adjusting the slag component are also charged from above the melting furnace (16). This method of loading from above facilitates maintenance of the charging equipment.
[0034]
Then, the unreduced iron oxide remaining in the reduced iron (15) is reduced by reacting (burning) the oxygen source (20) and the carbon material (18) supplied to the melting furnace (16). And heating and dissolving the reduced iron to produce molten iron having a carbon content of preferably 2% or more, more preferably 2.5 to 4.5%.
[0035]
At this time, supply of the oxygen source (20) and the carbonaceous material (18) such that the secondary combustion rate of the CO gas generated in the melting furnace (16) is 40% or less, more preferably 20% or more and 40% or less. By controlling the amount, the heat transfer efficiency of the secondary combustion heat to the molten metal is increased to 60% or more, more preferably 75% or more, and further preferably 80% or more. The reason why the secondary combustion rate is set to 40% or less and the heat transfer efficiency is set to 60% or more (more preferably, 75% or more) will be described in further detail later.
[0036]
As the oxygen source (20), high-purity oxygen having an oxygen concentration of preferably 90% or more is used, and the oxygen is blown upward, sideward, or bottomward toward the slag layer on the surface of the melting furnace (16). To stir the slag layer. If the oxygen is blown up or sideways, the maintenance of the blowing tuyere becomes easy and the melting furnace (16) does not need to be tilted. This is advantageous because it can be structured.
[0037]
The use of high-purity oxygen having an oxygen concentration of 90% or more facilitates control of the secondary combustion rate, and supplies reducing gas generated in the melting furnace (16) to the rotary hearth furnace (14). At the same time, it is easy to control the conditions necessary and sufficient to secure an appropriate level of gas calories, that is, a necessary theoretical combustion temperature.
[0038]
At this time, it is also effective to blow an inert gas (21) such as nitrogen into the molten iron for bottom-blowing stirring to enhance the stirring to promote the dissolution of the reduced iron (15).
[0039]
Note that a part or all of the carbon material (18) supplied to the melting furnace (16) and / or a carbon material different from the carbon material (18) supplied to the melting furnace (16) may be formed into a compact (12). ) Can be supplied directly to the rotary hearth furnace (14). This carbonaceous material may be supplied as a floor covering material on the hearth of the rotary hearth furnace (14), or the rotary hearth furnace (14) at the same time as the compact (12) using a device for supplying the compact (12). Or the compact (12) may be supplied after being supplied to the rotary hearth furnace (14). The carbonaceous material supplied to the rotary hearth furnace (14) is preferably in the form of powder when used as a floor covering material, but may be supplied to the rotary hearth furnace (14) simultaneously with the compact (12). When the compact (12) is supplied after being supplied to the rotary hearth furnace (14), it does not necessarily need to be powdery, and may be lumpy. When the carbonaceous material is supplied to the rotary hearth furnace (14) in this manner, the volatile matter in the carbonaceous material volatilizes and also functions as a heating raw material, so that the supply amount of the external fuel (17) can be reduced. preferable.
[0040]
In the above, another carbon material means that the carbon material (18) supplied to the melting furnace (16) and the furnace to be charged are different, or that another type of carbon material may be used. So, for example, when the carbon material (18) charged to the melting furnace (16) is coke, as another carbon material charged to the rotary hearth furnace (14), coal as a raw material of the coke is used. It is intended to be used. Therefore, different carbon materials do not mean completely different carbon materials.
[0041]
The carbon material is charred by being heated in the rotary hearth furnace (14) and then supplied to the melting furnace (16) as a carbon material (fuel) required for melting. When coal is used as the carbon material, the volatile matter of the coal is eliminated by charification in the rotary hearth furnace (14), and the coal is supplied as a preheated char to the melting furnace (16). Compared to the case of directly supplying (16) as carbon material (18), the amount of exhaust gas generated when charged into the melting furnace (16) is reduced, the exhaust gas equipment can be reduced, and the amount of excess exhaust gas (26) Is also preferred because it can also reduce.
[0042]
As the above carbon material (the same applies to the carbon material (18)), in addition to coal, wood chips, waste plastics, old tires, and the like, as well as coke, charcoal, and coke breeze that do not contain volatile components can be used. .
[0043]
A tap hole for taking out the molten iron (22) and the molten slag (23) is provided on the side wall of the melting furnace (16). The installation height of the tap hole is preferably set at a position where the stirring gas (21) does not blow through. Further, the melting furnace (16) has a sealable structure, and the whole or a part of the gas generated from the melting furnace (16) is supplied to the rotary hearth furnace (14) as a fuel source so that it can be effectively used. Good to do. In sending the gas generated from the melting furnace (16) to the rotary hearth furnace (14), the gas is once cooled as shown in the drawing, and the dust is removed through a dust removing device (24) to reduce the dust content to 5 g / Nm.ThreeDegree or less, desirably 1 g / NmThreeIt is better to reduce it below. As a result, the adhesion and deposition of dust on the inner walls of the gas pipe and the rotary hearth furnace (14) can be suppressed as much as possible. At this time, the sensible heat held by the high-temperature gas discharged from the melting furnace (16) is recovered by, for example, a radiant heat transfer boiler provided at the outlet of the melting furnace (16), and then the gas cooling / dust removing device (24 ) Is preferable because the sensible heat of the exhaust gas can be effectively utilized.
[0044]
Thereafter, the pressure is adjusted by a pressure booster (25), and then supplied to the combustion burner of the rotary hearth furnace (14). At this time, if the amount of the exhaust gas extracted from the melting furnace (16) as the fuel gas is excessive, the exhaust gas may be extracted outside as the excess exhaust gas (26) and effectively used as the fuel gas of the adjacent facility. If the melting furnace (16) has a closed structure and high-pressure oxygen gas is used, the pressure can be used to pressurize the inside of the melting furnace (16), and the pressure booster (25) can be omitted. .
[0045]
Since the gas discharged from the rotary hearth furnace (14) has little latent heat but is high temperature, heat is recovered by the waste heat boiler (27), and then the air is recovered by the air preheating heat exchanger (28). It can be used effectively for preheating of refuse. The exhaust gas heat recovered by the heat exchanger (28) is purified by a dust remover (30), and then is released to the atmosphere via a suction fan (31). The furnace pressure of the rotary hearth furnace (14) is controlled by the suction fan.
[0046]
The present invention is carried out in accordance with the flow chart as described above, and among these, the operating conditions of the rotary hearth furnace (14) and the melting furnace (16), which are particularly important, will be described in more detail.
[0047]
First, the rotary hearth furnace that is the main component of the reduced iron production facility will be described in detail. When a mixture containing an iron oxide-containing substance and a carbonaceous reducing agent, preferably a carbonaceous material-containing molded body formed by molding them is supplied to a rotary hearth furnace and heated, the following formulas (2) to (4)
FemOn + NC → mFe + nCO …… (2)
FemOn + NCO → mFe + nCOTwo …… (3)
C + COTwo → 2CO …… (4)
The reaction indicated by 進行 proceeds, and iron oxide is reduced. CO and CO generated hereTwoIs determined by the amount of the carbonaceous reducing agent contained in the molded article and the heating conditions.
[0048]
The raw material mixture charged on the rotary hearth is heated by combustion heat from burner combustion and radiant heat transfer from the furnace wall and ceiling. Since the heat radiation acts on the fourth power of the temperature, rapid temperature rise and reduction are possible, and iron oxide in the raw material mixture is reduced to metallic iron by heating for a very short time, for example, 6 to 12 minutes.
[0049]
The heat applied from the outer surface side of the raw material mixture is transmitted to the inside of the raw material mixture by conduction heat transfer, and the reactions of the formulas (2) to (4) are continued. In order to efficiently promote the heat transfer toward the inside of the mixture, the raw material mixture is formed into a compact, and its apparent density is set to 1.2 g / cm.ThreeAbove, desirably 1.8 g / cmThreeIt is desirable to keep the above.
[0050]
It is natural that the mixing ratio of the iron oxide source and the carbonaceous reducing agent should be such that the fixed carbon content excluding volatiles in the carbonaceous reducing agent is equal to or more than the chemical equivalent required for the reduction of iron oxide. It is preferable to determine the value by taking into account the amount of combustion heat required for heating and melting after being put into the melting furnace and the target carbon concentration equivalent of the molten iron generated by reduction melting.
[0051]
That is, the amount of the carbonaceous reducing agent and the amount of the carbonaceous material is such that the carbon amount (A) excluding the volatile matter in the carbonaceous stately reducing agent and the carbonaceous material is [(the chemical amount required for the reduction of iron oxide in the mixture. (Equivalent amount) + (target carbon concentration in molten iron product) + (heat amount required for dissolving solid reduced iron)], and the adjustment of the carbon amount (A) is performed by heat reduction. One selected from a carbonaceous reducing agent to be blended into the mixture charged into the furnace, a carbonaceous reducing agent to be blended into reduced iron produced in a heating reduction furnace, and a carbon material supplied to the melting furnace. What is necessary is just to carry out by the above amount. For example, when a large amount of carbonaceous material is provided at the stage of preparing the raw material mixture, the amount of carbonaceous material mixed into the solid reduced iron obtained by heat reduction may be reduced accordingly.
[0052]
Also, when performing reduction melting in a melting furnace, a CaO-containing substance is added to the melting furnace together with or separately from the solid reduced iron, and the basicity of the by-product slag is preferably adjusted to be 1.2 or more. preferable. By the way, if the basicity of the slag by-produced in the melting furnace is adjusted to 1.2 or more, the sulfur content in the molten iron moves toward the molten slag, and the sulfur content of the obtained metallic iron can be reduced. It is preferred.
[0053]
At this time, as the FeO content in the by-product slag decreases, the distribution ratio of the sulfur component in the slag direction increases, and the sulfur content in the molten iron decreases. Since the FeO content in the slag decreases as the amount of carbon (B) in the molten iron increases, the distribution of the sulfur component in the direction of the slag is increased to reduce the sulfur content in the molten iron. It is effective to increase the carbon content (B) in the medium to about 2% or more, more preferably to about 3% or more. It is preferable to reduce the amount of FeO in the slag in this manner, because melting of the refractory lining by the molten FeO can be suppressed.
[0054]
The carbon content (B) in the molten iron is
(1) a carbonaceous reducing agent blended into the raw material mixture charged into the heating reduction furnace;
(2) a carbonaceous reducing agent to be incorporated into the reduced iron produced in the heating reduction furnace,
(3) carbon material supplied to the melting furnace,
(4) Another carbon material charged to the heating reduction furnace
May be performed by any one or more of the above.
[0055]
By the way, as for the characteristics of the melting furnace in which reduced iron is reduced and melted, the metallization rate of the iron source (reduced iron) charged into the melting furnace is determined in order to efficiently promote the reduction and melting process in the melting furnace. The key is to keep it high, and for that purpose, it is important how to increase the metallization rate of reduced iron in a rotary hearth furnace.
[0056]
For this purpose, the conditions for raising and heating the raw material compact in the rotary hearth furnace must be appropriately and stably controlled, and the properties of the heating fuel gas should be maintained as stable as possible. When the gas generated in the melting furnace is sent to a rotary hearth furnace to be used as a fuel gas, the higher the calorie of the gas, the easier it is to raise the combustion temperature and the easier the temperature control of the rotary hearth furnace. This suppresses the secondary combustion rate in the melting furnace to a lowerTwoIt means that it is preferable to keep the content low. In addition, in order to stably burn the burner for a long time, minimize the dust in the fuel gas as much as possible, and prevent dust accumulation on the feed pipe and fuel gas burner and nozzle clogging as much as possible. It is desirable to do.
[0057]
Therefore, equipment is provided to cool the gas once and remove the dust before the exhaust gas from the melting furnace is led to the rotary hearth furnace. By this dust removal processing, the dust concentration in the gas is reduced to 5 g / Nm.ThreeBelow, desirably 1 g / NmThreeIt is recommended to reduce: The operating temperature of the dust removing equipment is preferably suppressed to about 800 ° C. or less in consideration of heat resistance and safety of the equipment.
[0058]
Next, the operating conditions of the melting furnace in which the solid reduced iron is reduced and melted will be described.
[0059]
The carbon material put into the iron bath in the melting furnace generates CO gas as shown by the following formula (5) by the reaction with the high-concentration oxygen injected at the same time.
C + 1 / 2OTwo → CO …… (5)
Secondary combustion occurs as shown in the following formula (6) in the gas phase on the iron bath.
[0060]
CO + 1 / 2OTwo → COTwo …… (6)
These reactions are exothermic reactions, and these heats are transferred to the iron bath and used as heat for further reducing and melting the solid reduced iron charged into the melting furnace.
[0061]
FIGS. 2 and 3 are graphs showing the relationship between the metallization rate of the iron source supplied to the melting furnace, the secondary combustion rate in the melting furnace, and the carbon material consumption. As is apparent from these figures, the carbonaceous material consumption decreases with an increase in the metallization rate of the iron source to be fed (FIG. 2) and decreases with an increase in the secondary combustion rate (FIG. 3).
[0062]
In particular, according to FIG. 2, when the secondary combustion rate is 40% or less, when the metallization rate is 60% or more, the carbon material consumption level off, and the fluctuation in the carbon material consumption due to the fluctuation in the metallization rate decreases. Therefore, it is extremely useful for performing a stable operation.
[0063]
Therefore, it is advantageous to increase the metallization rate of the iron source (that is, reduced iron) supplied to the melting furnace as much as possible in order to suppress the consumption of carbonaceous materials and promote stable operation, and at least 60% or more, more preferably It is desirably 80% or more, more preferably 90% or more, which is equivalent to ordinary iron scrap.
[0064]
In order to secure a metallization ratio of 60% or more, the amount of the carbonaceous reducing agent blended during the production of the raw material mixture and the conditions for heating and reducing in a rotary hearth furnace may be appropriately controlled. In addition to blending a carbonaceous reducing agent in a necessary and sufficient amount for the reduction of iron oxide as described above in the preparation of the raw material mixture, the operating temperature in the rotary hearth furnace is set to 1100 to 1400 ° C, more preferably 1250 to 1350 ° C. The residence time should be at least 6 minutes, more preferably at least 8 minutes.
[0065]
From FIG. 3, it is desirable to increase the secondary combustion rate, more preferably to increase the secondary combustion rate in the melting furnace, in order to effectively exhibit the carbon material consumption reduction effect in the actual operation. % Or more should be secured. However, if the secondary combustion rate exceeds 40%, the effect of further reducing the carbon material consumption is hardly recognized, so the secondary combustion rate is preferably suppressed to 40% or less, more preferably to about 30% or less.
[0066]
Since the secondary combustion rate varies depending on the amount of carbon material added to the melting furnace, the amount of oxygen gas injected, and the like, in order to control the value to 40% or less, more preferably in the range of 20 to 40%, The amount of added carbonaceous material and the amount of injected oxygen gas may be appropriately controlled in consideration of the secondary combustion rate.
[0067]
Further, the secondary combustion raises the temperature of the gas phase side in the melting furnace, and gives a great heat load to the refractory lining. Decreasing the metallization rate of the input iron source means that the amount of unreduced iron oxide (FeO) contained in the iron source increases, and thus the amount of FeO in the by-product slag increases. This accelerates the erosion of the refractory lining. Therefore, as described above, water-cooling of the refractory has been attempted as a means for suppressing erosion of the refractory, but in this method, the heat loss due to the water-cooling has a significant effect on production efficiency and cost.
[0068]
In order to promote the dissolution of the iron source (reduced iron) added to the melting furnace, stirring the iron bath is effective. However, if the stirring is strengthened, the amount of dust in the exhaust gas discharged from the melting furnace is reduced. Increase (up to 100 g / NmThreeTo the extent), not only lowers the yield of iron, but also causes adhesion and deposition in the hot gas pipes, causing blockage accidents.
[0069]
In view of the above, in the present invention, the carbonization rate is reduced by increasing the metallization rate of the reduced iron supplied to the melting furnace to 60% or more, and more preferably to 80% or more. By suppressing the secondary combustion rate to 40% or less, more preferably about 20 to 40%, and still more preferably 20 to 35%, thereby avoiding an excessive rise in the gas phase temperature and reducing the load on the melting furnace. I am trying to reduce it.
[0070]
Air can be used as the oxygen source to be blown into the melting furnace. In that case, however, nitrogen containing about four times the amount of oxygen is also preheated, and waste of preheating energy increases. However, the amount of generated gas also increases. Therefore, in order to increase the thermal efficiency and avoid an unnecessary increase in the amount of generated gas, it is desirable to use high-purity oxygen, preferably high-purity oxygen having an oxygen purity of 90% or more, as an oxygen source. And the amount of dust generated can be reduced.
[0071]
Next, the relationship between the secondary combustion rate and the heat transfer efficiency in the melting furnace and the temperature of the exhaust gas from the melting furnace were investigated, and the results of comparison with the conventional example are shown in FIG.
[0072]
As is clear from FIG. 4, if the heat transfer efficiency is constant, the exhaust gas temperature rises as the secondary combustion rate increases, and the amount of heat discharged without being effectively used in the melting furnace increases. Conversely, if the exhaust gas temperature can be kept constant, the heating efficiency increases as the secondary combustion rate increases, and it can be confirmed that heat is used effectively. Case A shown in FIG. 4 is an example in which scrap is used as the iron source to be fed into the melting furnace. The secondary combustion rate is 20%, the heat transfer efficiency is as high as 89%, and the exhaust gas temperature is about 1650 ° C. Low results have been obtained.
[0073]
On the other hand, Case B uses reduced iron having a metallization rate of 30% as the iron source to be fed into the melting furnace. Since the secondary combustion rate has increased to about 45%, the exhaust gas temperature is 1900 ° C. As the temperature rises, the heat load on the refractory lining increases, and the heat radiating efficiency decreases to 85%. Furthermore, in this case B, since the metallization rate of the input iron source is as low as 30%, the (FeO) concentration in the slag by-produced during reduction and melting increases, and the erosion of the lining refractory is accelerated. confirmed.
[0074]
From these results, as a desirable condition when operating an integrated facility that connects a heating and reducing device using a rotary hearth furnace and a melting furnace that reduces and melts the generated reduced iron,
{Circle around (1)} The metallization rate in a rotary hearth furnace is increased to 60% or more, more preferably 80% or more to minimize residual (FeO).
{Circle around (2)} In order to secure the calories necessary for using the exhaust gas from the melting furnace as fuel for the rotary hearth furnace, the secondary combustion rate in the melting furnace should be 40% or less, more preferably 20 to 40%. Control over the range,
(3) The secondary combustion rate should be reduced to 40% or less in order to prevent melting of the refractory lining of the melting furnace and to prevent the exhaust gas temperature from becoming excessively high.
Is important, and the shaded region in FIG. 4 is recommended as a preferable condition.
[0075]
That is, as described with reference to FIGS. 2 and 3, the metallization ratio of the reduced iron charged into the melting furnace after the heat reduction in the rotary hearth furnace is increased to 60% or more, and the reduction iron is generated in the melting furnace. The supply amounts of oxygen and carbon material to the melting furnace are controlled so that the secondary combustion rate of the CO gas is 40% or less, and the heat transfer efficiency of the secondary combustion heat to the molten iron is 60% or more, more preferably. Has been confirmed to be increased to 75% or more.
[0076]
The heat transfer efficiency (Ef) of the secondary combustion heat to the molten iron is defined as follows.
[0077]
Ef = [1- (HThree+ HFour-HTwo) / H1] X 100 (%)
H1: Calorific value of secondary combustion reaction. Here, the secondary combustion reaction refers to CO, H generated from molten iron.TwoThe combustion of gas with oxygen is represented by the following reaction formula.
[0078]
CO + (1/2) OTwo= COTwo
HTwo+ (1/2) OTwo= HTwoO
HTwo: Sensible heat of gas generated from molten iron. The gas amount and composition are calculated from the material balance of the molten iron, and the temperature is the same as that of the molten iron.
[0079]
HThree: Sensible heat of gas discharged from the furnace.
[0080]
HFour: Heat loss from the gas phase side where the secondary combustion reaction occurs (corresponding to 10 to 20% of the total heat input).
[0081]
If these conditions are met, the life of the refractory lining of the melting furnace will be extended, so there is no need to make the melting furnace tiltable or movable for intermediate repairs and maintenance. Even in this case, it is possible to continue the operation without any trouble for a long time. However, the present invention is not limited to the use of a fixed type melting furnace, and it is of course possible to use a rolling type melting furnace.
[0082]
Thus, according to the present invention, the raw material mixture containing the carbonaceous reducing agent is charged into a heating reduction furnace such as a rotary hearth furnace, and the iron oxide in the mixture is reduced to solid reduced iron. To the melting furnace to further reduce by heating and melt the reduced iron to produce molten iron,
a) Increase the metallization rate of solid reduced iron to 60% or more in a heating reduction furnace,
b) controlling the oxygen supply amount and the carbon material supply amount so that the secondary combustion rate of the CO gas generated in the melting furnace is 40% or less;
c) Preferably, the heating efficiency of the combustion heat by the secondary combustion to the molten metal is increased to 60% or more,
d) by providing the melting furnace with a closed structure, supplying all or a part of the gas generated from the melting furnace to the heating reduction furnace as fuel, and heating the obtained solid reduced iron in the melting furnace,
It has become possible to produce a reduced iron melt having a carbon content of about 1.5 to 4.5% with high energy efficiency and high productivity while minimizing deterioration of the heating reduction furnace and the melting furnace. .
[0083]
【Example】
Hereinafter, the present invention will be described more specifically with reference to Examples. However, the present invention is not limited to the following Examples, and may be appropriately modified within a range that can conform to the purpose of the preceding and the following. The present invention can be implemented, and all of them are included in the technical scope of the present invention.
[0084]
Example
Based on the process flow chart shown in FIG. 1, a test operation was performed under the conditions shown in Table 2 using raw ores and coal having the chemical compositions shown in Table 1, and the results shown in Table 2 were obtained.
[0085]
[Table 1]
[Table 2]
In Table 2, No. Controls 1-3 are such that the metallization rate of reduced iron produced in the rotary hearth furnace is kept at 90%, the secondary burning rate is 40% or less in the melting furnace, and the heating efficiency is 60-90%. No. Reference numeral 1 denotes a case in which the entire amount of gas generated from the melting furnace is introduced into the rotary hearth furnace, and the heat shortage is supplemented with auxiliary fuel (in this example, natural gas is used).
[0088]
No. No. 2 is an example in which the heating efficiency of the melting furnace is increased and the amount of generated gas is increased to aim at zero auxiliary fuel in the rotary hearth furnace. As a result, the amount of dust in the exhaust gas from the melting furnace slightly increases, but does not hinder the actual operation. Further, it can be seen that the amount of exhaust gas from the melting furnace is slightly excessive and can be used as an external heat source, albeit slightly.
[0089]
No. No. 3 is an example of optimizing all the process parameters, using no auxiliary fuel in the rotary hearth furnace, and simultaneously aiming for no surplus gas from the melting furnace. A self-contained operation has been established.
[0090]
On the other hand, no. In No. 4, the secondary combustion efficiency is kept low at 30%, but the heat transfer efficiency to the molten iron in the melting furnace is slightly low at 73%. Also, the generation amount and dust concentration tend to increase slightly. No. No. 6 is an example in which the amount of carbonaceous material charged into the melting furnace was increased to increase the amount of carburizing, and the carbon content of the molten iron was increased to the saturated carbon content level. That is, according to the present invention, it is easy to increase the carbon amount of the molten iron to the saturated carbon concentration level by adjusting the amount of carbon blown into the melting furnace.
[0091]
No. 5 is an example in which the secondary combustion rate in the melting furnace is excessively increased, and the heating efficiency is increased. However, since the amount of exhaust gas and the amount of heat (reduction potential) sent to the heating reduction furnace are reduced, the rotation speed is reduced. The hearth furnace required additional cooking with auxiliary fuel.
[0092]
As is evident from these results, optimizing the above operating conditions stabilizes a series of operations from solid reduction to reduction melting without applying excessive heat load to the melting furnace with high energy efficiency. And efficiently perform, and can produce high-purity molten iron with high productivity. Then, for example, the above No. As shown in 3, if the operating conditions are well controlled, a self-contained operation in terms of energy can be realized in a series of molten iron production facilities.
[0093]
Note that the above No. In the production of molten metallic iron under the same conditions as in Example 3, the calcined lime (CaO) was additionally charged into the melting furnace together with the carbonaceous material for heating, so that the basicity (CaO / SiOTwoRatio) was adjusted to be in the range of 1.5 to 1.6, while reducing and melting were continued, and the S content of the obtained molten iron was measured. As a result, the S content gradually increased in the initial stage of the start of operation, and after 40 minutes, the S content increased to about 0.04% by mass, but the S content did not increase thereafter, and the obtained metallic iron Was stable at about 0.04% by mass. This is considered to be because S in the molten metal moved in the slag direction by increasing the basicity of the generated slag by additionally adding quicklime to the melting furnace.
[0094]
【The invention's effect】
As described above, according to the present invention, it is possible to efficiently produce molten iron with a smaller amount of energy consumption as compared with the conventional method, and furthermore, the refractories are less worn, the energy flexibility is high, and the production elasticity is high. Can provide iron making process.
[Brief description of the drawings]
FIG. 1 is a process flow chart showing one embodiment of the present invention.
FIG. 2 is a graph showing a relationship between a metallization ratio of iron supplied to a melting furnace and a carbon material consumption when a secondary combustion rate in the melting furnace is changed.
FIG. 3 is a graph showing the relationship between the amount of carbonaceous material consumed and the secondary combustion rate when the metallization ratio of iron charged into a melting furnace is changed.
FIG. 4 is a graph showing the relationship between the efficiency of heating the iron bath in the melting furnace and the secondary combustion rate when the temperature of the exhaust gas from the melting furnace is changed.
Claims (23)
前記固形還元鉄の金属化率を６０％以上に高めてから溶解炉へ送り、該溶解炉へ供給する酸素と炭材の量を制御することによって、該溶解炉内におけるＣＯガスの２次燃焼率を４０％以下に制御することを特徴とする溶鉄の製法。A raw material mixture containing an iron oxide source and a carbonaceous reducing agent is charged into a heating reduction furnace, and the iron oxide in the raw material mixture is reduced by a carbonaceous reducing agent into solid reduced iron, and the solid reduced iron is dissolved. In the method for producing molten iron, the solid reduced iron is melted in the melting furnace to obtain a molten iron, by feeding to a furnace and burning a carbon material supplied as fuel.
Secondary combustion of CO gas in the melting furnace by controlling the amount of oxygen and carbon material supplied to the melting furnace after increasing the metallization rate of the solid reduced iron to 60% or more and controlling the amount of oxygen and carbon material supplied to the melting furnace. A method for producing molten iron, wherein the rate is controlled to 40% or less.
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Cited By (7)
| Publication number | Priority date | Publication date | Assignee | Title |
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| WO2006046606A1 (en) * | 2004-10-29 | 2006-05-04 | Kabushiki Kaisha Kobe Seiko Sho | Process for producing molten iron and apparatus therefor |
| JP2006257545A (en) * | 2005-02-18 | 2006-09-28 | Kobe Steel Ltd | Method for producing molten iron and apparatus therefor |
| JP2007077452A (en) * | 2005-09-14 | 2007-03-29 | Nippon Steel Corp | Method for utilizing dust into steelmaking in converter and method for operating converter exclusive for dust melting |
| WO2007099941A1 (en) * | 2006-03-01 | 2007-09-07 | Kabushiki Kaisha Kobe Seiko Sho | Melting furnace and process for producing molten metal therewith |
| WO2009038139A1 (en) | 2007-09-19 | 2009-03-26 | Kabushiki Kaisha Kobe Seiko Sho | Process for producing molten iron |
| WO2009038140A1 (en) | 2007-09-19 | 2009-03-26 | Kabushiki Kaisha Kobe Seiko Sho | Process for producing molten iron |
| US7993430B2 (en) | 2006-04-25 | 2011-08-09 | Kobe Steel, Ltd. | Process for producing molten iron and apparatus for producing molten iron |
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Cited By (10)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2006046606A1 (en) * | 2004-10-29 | 2006-05-04 | Kabushiki Kaisha Kobe Seiko Sho | Process for producing molten iron and apparatus therefor |
| JP2006257545A (en) * | 2005-02-18 | 2006-09-28 | Kobe Steel Ltd | Method for producing molten iron and apparatus therefor |
| JP2007077452A (en) * | 2005-09-14 | 2007-03-29 | Nippon Steel Corp | Method for utilizing dust into steelmaking in converter and method for operating converter exclusive for dust melting |
| WO2007099941A1 (en) * | 2006-03-01 | 2007-09-07 | Kabushiki Kaisha Kobe Seiko Sho | Melting furnace and process for producing molten metal therewith |
| US7993430B2 (en) | 2006-04-25 | 2011-08-09 | Kobe Steel, Ltd. | Process for producing molten iron and apparatus for producing molten iron |
| US8277536B2 (en) | 2006-04-25 | 2012-10-02 | Kobe Steel, Ltd. | Process for producing molten iron and apparatus for producing molten iron |
| WO2009038139A1 (en) | 2007-09-19 | 2009-03-26 | Kabushiki Kaisha Kobe Seiko Sho | Process for producing molten iron |
| WO2009038140A1 (en) | 2007-09-19 | 2009-03-26 | Kabushiki Kaisha Kobe Seiko Sho | Process for producing molten iron |
| US7993428B2 (en) | 2007-09-19 | 2011-08-09 | Kobe Steel, Ltd. | Method for manufacturing molten iron |
| US8012237B2 (en) | 2007-09-19 | 2011-09-06 | Kobe Steel, Ltd. | Process for producing molten iron |
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